Parathyroid hormone-related peptide (PTHrP) is a member of the parathyroid hormone (PTH) family, sharing a close homology with PTH at the N-terminal sequence. Because both hormones bind to the same receptors, various physiological functions of PTHrP are similar to those of PTH. PTHrP functions as an autocrine, paracrine, and endocrine hormone, and is able to simulate most of the actions of PTH, including regulation of calcium ion homeostasis, bone resorption, distal tubular Ca reabsorption, and inhibition of proximal tubular phosphate transport. In health, PTHrP regulates bone development by maintaining the endochondral growth plate. It also plays a role during tooth eruption, development of mammary glands, pregnancy, fetal development, and regulation of Ca transfer to milk during lactation. Circulating concentrations of PTHrP in health are very low but can be increased during pregnancy and lactation and in some nonmalignant diseases.